A laser scan consists of a large number of range and intensity measurements made from a single location in space. By steering the laser beam through horizontal and vertical angles, distances are measured to objects visible within the line-of-sight. Each point is measured in the coordinate frame of the scanner itself. Because of occlusion and the finite performance range of the scanner, it is necessary to scan from multiple vantage points throughout the scene in order to provide complete and accurate coverage. For each scan taken, a new coordinate frame is established unique to location and orientation of the scanner during operation.
When using more than one scan, it becomes necessary to determine the geometric information connecting these coordinate frames together. With this information, it becomes possible to relate measurements made among scans to others or to the world coordinate system. This process of connecting coordinate frames together is called registration. In fact, registration is often necessary even if only one scan is taken because the subsequent processing of the data requires it to be referenced back to a world coordinate system established in the scene itself.
One common approach has been to structure the scene with targets, which are then surveyed into the world coordinate system. Each scan then can be registered to the world system by identifying the targets visible and determining the rotation and translation parameters that best align scan with the network of targets. Essentially an independent survey network is created, into which the scans are registered one-by-one.
The advantages of the survey approach are that it produces results of high accuracy and also quantifies the uncertainty of these results. The disadvantages, however, is that measurements are error-prone and difficult to make, and overall the process is slow. Consequently, blunders are difficult to identify. Improvements are possible if more targets (and therefore) more measurements per setup station are used, but doing so further slows down the process.